I don't have a copy of Mandl to compare, but I'm looking at Weinberg section 6.1 where he says something similar. He's evaluating the S-matrix as a sum of terms, <0| ... |0> where ... is a string of creation and annihilation operators, and he's talking about rearranging the order of the operators. Every time you switch the order of two of them you get a numerical factor.

And on p262 he says: (f) Pairing of a field ψ with a field adjoint ψ† in H(y) yields a factor -iΔ(x,y). (I'm leaving some subscripts out.) This is close to what you're saying. So I think the context is that Mandl's Eq (1) represents a subexpression that's eventually going to be placed between <0| |0>'s.